The present invention relates to an improved epicyclic or planetary gear comprising a sun pinion fixed to a first shaft rotatably mounted through bearings in the gear housing, a hollow wheel concentric with said sun pinion and at least two planet wheels meshing with said sun pinion and said hollow wheel, and interconnected by a planet carrier rotatably fixed to a second shaft coaxial to said first shaft.
An epicyclic gear of this type is already known through the West-German patent application No. DE-OS 25 58 093 depicting an epicyclic gear in which the planet carrier and its planet wheels are axially fixed whereas the sun pinion and the internally toothed annulus are axially movable and urged by an axially directed force, notably that of spring means. All wheels are provided with a profile correction changing continuously along the tooth width. Through the continuously changing profile correction of the spur teeth the wheels are allowed to slip axially in relation to one another until they mesh with each other substantially without any play, thus compensating the work tolerances. In addition, to obtain a load compensation or balance, the axially movable wheels are also movable radially within narrow limits. For this purpose, the sun pinion is adapted to be driven through a coupling sleeve formed at its two end portions with inner splines meshing on the one hand with corresponding outer splines formed on a stub shaft rigid with the sun pinion and on the other hand with corresponding outer splines formed likewise on the drive shaft. Disposed between this drive shaft and the stub shaft is a compression spring. With this bearing arrangement, the sun pinion is not only allowed to slide in relation to the drive shaft or the gear housing, but also to move radially, obviously on account of the very moderate resiliency of the coupling sleeve. In a similar manner, the annulus is provided with outer splines meshing with the inner splines of a special coupling sleeve meshing in turn with outer splines formed on an annular projection of the gear housing.
In this known epicyclic gear the sun pinion and the large, heavy annulus must unfavorably be mounted for both axial and radial movement by using special coupling sleeves provided with inner splines, so that a rather complicated construction is obtained. Furthermore, it is more difficult to achieve the desired self-adjustment due to the fact that the heavy mass of the annulus must be capable of moving axially and/or radially, that is, on the whole, in a rapid periodic sequence depending on the rotational speed of the planet wheels. The therefore necessary periodic accelerations of a relatively heavy inert load are a source of undesired losses and stresses, and may eventually lead to detrimental vibrations.
In addition, two further power transmissions through teeth or splines are introduced, and it is necessary to take due account of, and possibly eliminate, the play developing therein, if it is desired not to interfere with the free working play to be obtained through the continuously changing profile correction of the teeth of the sun pinion, planet wheels and annulus. Furthermore, it is scarcely possible to compensate an untrue running of this known gear construction. It is also notoriously precluded, in the present state of the art, to attain the desired perfect load balance, since the distance between the planet wheels or the radial positions of all the planet wheels have fixed values that cannot be changed. For these various reasons, epicyclic gears of the type mentioned above have not been used in actual practice so far.
Epicyclic gears with bevel wheels and pinions are also known in the art (cf. West German Pat. No. DE-OS 2 363 106), wherein the planet wheels are resiliently urged in the axial direction and rotatably supported by the planet-carrier stud shafts. With this method and by using normal working tolerances it is possible to obtain a torque distribution. The radially rigid planet wheels are provided in this case with two opposed sets of bevel teeth, thus obviously increasing the manufacturing cost and the axial dimensions of the gear.
On the other hand, epicyclic gears are known wherein, for obtaining the best possible load balance, the planet wheels are mounted on spherical or pendulum bearings (cf. West-German patent application No. DE-PS 503 307 and U.S. Pat. No. 3,178,966).
Notwithstanding the multiplicity of known epicyclic gears which, on account of their space-saving and compact construction, their coaxially arranged driving and driven shaft and the multiple possibilities of changing their reduction ratios in a simple way, up to now one had to renounce their application whenever a substantially play-free power transmission was required. This applies before all to actuators, more particularly to follower controls and the like, in which an absolutely accurate positioning is a must, as for example in handling and manipulating machines and apparatus, especially for driving industrial robots, and also for the digital control of precision machine tools, the position control of astronomical observation instruments, and the like. In such cases, even a lateral play of only a few arc minutes may already lead to unacceptable bearing defects. Consequently, up to now for such applications special gears having little or no play at all and consistent with each specific use were used, but this proved to be an expensive and awkward solution, as for example the cycloid disk gear, the special spur-gear with precision-selected wheels, the spur-gear with torsion bracing or the like. The last-named gear construction is also objectionable in that, on account of the torsion bracing, its resiliency is exerted in the circumferential direction, whereby mainly during strong accelerations and quick changes in the direction of rotation unacceptable vibrations are most likely to develop.